1. Field of the Invention
The present invention relates to a resin-coated metal plate suitable as a cover plate for efficiently obtaining a high-quality product in a perforating process as one of the production processes for a printed-wiring board, and a printed-wiring board production method including a perforating process using the resin-coated metal plate.
2. Description of the Related Art
A printed-wiring board plays a role in electrically connecting between a plurality of electronic components mounted (or packaged) thereon, and its role as an internal component for electric products is of significant importance. As one of the processes for producing such a printed-wiring board, there has been known a perforating process for forming a through-hole in a multilayer printed-wiring board in its cross-sectional direction so as to provide an electrical conduction between the uppermost and lowermost layers of the board. In addition to the essentiality of the through-hole to the printed-wiring board, it is generally required to form the through-hole in a large number. Therefore, the perforating process occupies an important position in the production of printed-wiring boards.
Typically, a printed-wiring board comprises a composite material with a laminated structure which includes a conductive layer formed of a copper foil, and an insulative layer prepared by impregnating a cross-web of woven glass fibers with epoxy resin and curing the epoxy resin. The respective materials of these layers are different in physical properties, and thereby it is liable to cause defects, such as boundary separation between the layers or cracks, during drilling, which undesirably lead to deterioration in surface roughness of the inner wall of a perforated hole, and wearing or breakage of a drill. In addition, the front surface of the printed-wiring board having periodic irregularities derived from dents in the glass-fiber cross-web is liable to cause deterioration in positional accuracy of a drilled hole during a perforating process using a drill. Moreover, the resin in the printed-wiring board is softened due to heat generated during the perforating process using a drill, and adhered to the inner layer or copper foil in the form of a stain, so-called drill smear, which will be one of factors causing defective electrical conduction after plating of through-holes.
Particularly, in connection with downsizing and elaboration in electronics devices, a perforated hole to be formed in a printed-wiring board is progressively reduced in diameter, and increased in density or narrowed in hole pitch. Thus, it becomes essential to provide technologies allowing a perforated hole to be formed at a more accurate position with an inner wall having enhanced surface smoothness. Further, in view of the need for cutting a production cost due to recent economic circumstances, it is strongly urged to establish more efficient production technologies for printed-wiring boards.
As a technique for suppressing heat generation during perforating to solve the above problem, U.S. Pat. Nos. 4,781,495 and 4,929,370 disclose a method of perforating a multilayer printed-circuit board with a drill, through the use of a cover plate comprising a water-soluble-lubricant-impregnated sheet disposed on one or both of the opposite surfaces of the multilayer board. The lubricant-impregnated sheet for use in this perforating method is prepared by impregnating a porous material, such as paper, with a mixture of glycol, such as diethylene glycol or dipropylene glycol, which is a solid water-soluble lubricant, a synthetic wax, such as fatty acid, and nonionic surfactant.
Japanese Patent Laid-Open Publication No. 2002-120198 discloses a method of perforating a multilayer printed-circuit board with a drill, through the use of a cover plate comprising a water-soluble resin-coated metal plate prepared by adhering a mixture a polycondensation product of polyethylene glycol and dimethyl terephthalate, and polyoxymethylene monostearate, which are a water-soluble compound onto one of the surfaces of the aluminum plate.
An improvement of the above techniques includes a method of perforating a multilayer printed-wiring board while disposing a thermoplastic film having a melting temperature of 100° C. to 150° C., directly on the copper foil. (Japanese Patent publication No. 07-050831) Other improvement includes a method of perforating a printed-wiring board while stacking a sheet on the printed-wiring board. The sheet is prepared by bonding a metal foil and a thermoplastic resin film with a multilayer structure including a lubricant-containing thermoplastic resin layer and a no-lubricant thermoplastic resin layer laminated on the outer surface of the lubricant-containing thermoplastic resin layer. (Japanese Patent Laid-Open Publication No. 2001-150215) Further, a method using a sheet with a laminated structure including a metal foil and a polyester resin composition prepared by mixing inorganic filler and/or water-soluble lubricant with thermoplastic polyester resin is proposed. (Japanese Patent Laid-Open Publication Nos. 2001-246696 and 2000-218599).
However, none of the above techniques provides sufficient advantages to the need for forming a hole at a more accurate position with an inner wall having enhanced surface smoothness, in the process of perforating a printed-wiring board with a drill.
For example, the aforementioned method of perforating a printed-circuit board with a drill, through the use of a cover plate comprising a water-soluble lubricant-impregnated sheet or a water-soluble resin-coated metal plate disposed on the printed-circuit board, can definitely offer a certain level of advantage in preventing the heat generation in the drill, providing reduced roughness in the inner surface of a perforated portion of the printed-wiring board, and achieving extended life of the drill. On the other hand, the method has disadvantages in insufficient impregnation of the mixture serving as an essential component of the sheet to the porous material, deterioration in the adhesion between the metal plate and the water-soluble resin, occurrence of warping or bending in the resin-coated metal plate, and poor handling or operating performance due to surface tackiness of the resin-coated metal plate, particularly under high-temperature and high-humidity conditions, such as rainy season or summer season, caused by sticky properties of the film itself. The inventor's experience in the researches for improving these problems by means of the addition of inorganic powder filler has shown that the tackiness of the film caused by moisture absorption or exposure to water is particularly an intrinsically inevitable problem in the technique using a film comprising a primary component of water-soluble resin.
In the aforementioned technique of mixing lubricant with thermoplastic resin, even if it is designed such that a no-lubricant thermoplastic resin layer is additionally laminated on the bonded portion of the lubricant-containing resin layer to the metal foil, it will be liable to cause deterioration in the adhesion between the additional resin layer and the metal foil, because the lubricant will be diffused in the bonded portion between the additional resin layer and the metal foil, with time. As a result, chips formed during perforating are likely to enter between the additional resin layer and the metal foil so as to deteriorate the positional accuracy of a perforated hole, and a drill is likely to be broken due to insufficient discharge of the chips.
The thermoplastic resin disclosed in the aforementioned Japanese Patent Laid-Open Publication No. 2001-150215 has a high hardness, which leads to significant deterioration in positional accuracy of a perforated hole due to slipping of a drill onto the board during perforating.
Accordingly, as a solution of the above problems, a coated resin which has properties suitable for a perforating process using a drill has been desired.